Touch
- distinctive from the other 4 senses in 2 ways
- receptors for the somatic sensation are distributed throughout the body
- somatic sensory system processes many types of stimuli and the evoked sensations are diverse
- 4 distinct somatic modalities
- touch
- proprioceptive sensations
- pain
- thermal sensations
- Skin
- the interface between the organism and its environment
- complex structure
- hairy skin
- glabrous skin
- epidermis
- outer layer of dead cells over a layer of living cells
- dermis
- inner layer
- contains nerve endings of the skin
- different types of mechanoreceptors in the skin mediate touch
- slowly adapting -- respond continuously to a persistent stimulus
- rapidly adapting -- respond to the onset and termination of the stimulus
- glabrous skin
- very sensitive to touch
- Meissner's corpuscle -- rapidly adapting
- Merkel's receptor -- slowly adapting
- both have small receptive fields - 2-4mm
- subcutaneous tissue has 2 types of mechanoreceptors
- pacinian corpsule -- rapidly adapting
- Ruffini's corpusle -- slowly adapting
- both have large receptive fields
- difference between the 2 types of rapidly adapting receptors is how they respond to vibrations
- Meissner's -- low frequency stimulation (fluttering of the skin)
- pacinian -- high frequency stimulation (deeper humming sensation)
- complex natural stimuli usually activate multiple receptor types
- Merkel's receptor - encodes fine spatial details
- Meissener's and pacinian corpusles probably provide timing information for tactile discriminations under active conditions
Central somatic pathways
- 2 major pathways to the somatosensory cortex
- dorsal column-medial lemniscal system -- tactile sensation and arm proprioception
- anterolateral system -- pain and temperature a bit of tactile information
- the body surface is represented in the brain in an orderly fashion
- S1 postcentral gyrus
- forms a map of the body - homunculus
- cortical map represents the contralateral side of the body
- each part of the body is represented in the brain in proportion to its importance in sensory perception
- neurons in somatosensory cortex have a receptive field that corresponds to specific regions of the skin
- touching a region on the skin stimulates the neurons connected to the area -- Mountcastle
- stimulating a small group of neurons in the somatosensory cortex evokes a sensation of being touched on a specific part of the skin -- Penfield
Detection and discrimination of touch
- some parts of the skin are more sensitive
-
| Body area (left) |
Two point
threshold (cm) |
Inverse |
| chin |
0.55 |
1.82 |
| above upper lip |
0.3 |
3.33 |
| scalp |
5.4 |
0.19 |
| forehead |
1.9 |
0.53 |
| nose (bridge) |
0.9 |
1.11 |
| eyelids |
0.65 |
1.54 |
| cheek |
1.60 |
0.63 |
| lower lip |
0.3 |
3.3 |
| upper lip |
0.25 |
4.00 |
| ear |
2.40 |
0.42 |
| back neck |
1.35 |
0.74 |
| side neck |
1.85 |
0.54 |
| front neck |
2.55 |
0.39 |
| top shoulder |
4.4 |
0.23 |
| outer upper arm |
16.3 |
0.06 |
| inner upper arm |
5.2 |
0.19 |
| outer forearm |
5.2 |
0.19 |
| inner forearm |
4.6 |
0.22 |
| back of hand |
1.7 |
1.25 |
| second finger front |
0.25 |
4.00 |
| second finger back |
0.25 |
4.00 |
| thumb back |
0.75 |
1.33 |
| thumb front |
0.3 |
3.33 |
| back |
4.2 |
0.24 |
| chest |
3.8 |
0.26 |
| thigh outer |
3.65 |
0.27 |
| thigh inner |
3.35 |
0.30 |
| lower leg front |
4.0 |
0.25 |
| lower leg back |
8.95 |
0.1 |
| foot top |
4.6 |
0.22 |
| foot bottom |
3.3 |
0.3 |
| middle toe top |
3.05 |
0.33 |
| big toe top |
2.6 |
0.38 |
- we are most sensitive to low frequency vibrating stimulus
- we can detect a stimulus that evokes 1 impulse in 1 rapidly adapting fiber from the hand
- we can localize a touch to a particular region of the skin
- our sensitivity to touch can be studied by measuring our ability to discriminate 2-points
Proprioception - Kinesthesis
- sensory receptors that signal the position and movement of the parts of the body
- the ability to move a limb and to feel the movement are separate
- we can sense to position of a limb even when it is moved passively
- receptors associated with the muscles, joints, and skin provide proprioceptive information
- perception of joint angle determined mostly by detecting the length of the muscle by the muscle spindle receptor
- muscle spindle fiber fires at high frequency when the muscle is stretched -- stimulating the fiber to fire at a high frequency gives the perception of extention of the limb
- proprioception can adapt
Copyright © Kathryn M. Murphy1996-05
